Health concerns are raised over the imported wallboard. Some drywall made with radioactive phosphogypsum, a waste byproduct, was shipped to the U.S. by at least four Chinese-based firms.

WUHAN, CHINA, AND LOS ANGELES — The final years of the U.S. housing boom and a disastrous series of Gulf Coast hurricanes created a golden opportunity for Chinese drywall manufacturers. With domestic suppliers unable to keep up with demand, imports of Chinese drywall to the U.S. jumped 17-fold in 2006 from the year before.

That imported drywall is now at the center of complaints of foul odors seeping from walls. Hundreds of homeowners, most in Florida, have also reported corrosion to their air conditioners, mirrors, electrical outlets and even jewelry.

State and federal authorities have traced the problems to Chinese-made drywall but haven't yet fully determined the causes. Some Chinese experts, however, suspect that the culprit is a radioactive phosphorus substance -- phosphogypsum -- that is banned for construction use in the U.S. but has been used by Chinese manufacturers for almost a decade.

Copies of Chinese customs reports obtained by The Times, along with interviews, indicate that drywall made with phosphogypsum was shipped to the U.S. in 2006 by at least four Chinese-based manufacturers and trading firms.

The health risk of phosphogypsum is uncertain, but industry specialists say they are troubled by its widespread use and the possibility it was exported, especially in light of recent incidents in which other Chinese imports such as pet food, toys and candy were found to be contaminated with toxic or unsafe substances.

"Considering the fact that phosphogypsum can cause corrosion, something should be done," said Ding Dawu, a geoscientist and an authority on gypsum processing in China. "Right now," he added, "there are no complaints [in China] because most people don't know much about gypsum board and there are no standards against it."

The Times contacted about 20 Chinese-based companies involved in making or selling drywall. Most of them declined to comment about their overseas business. Others offered conflicting answers or said they didn't know whether their drywall contained the banned substance.

One exception was Beijing Building Materials Import & Export Co. It was among at least 10 trading firms that exported drywall to the U.S. in 2006, according to the customs reports, which were provided by an industry source and confirmed by the Chinese government's statistics compiler in Hong Kong.

The reports show that Beijing Building Materials shipped nearly 38 million pounds of drywall to the U.S. in 2006. Sun Yong, the company's vice president, said it didn't matter whether the wallboard was made with mined gypsum or phosphogypsum.

"From China's customs side, there is no special inspection of exported drywall," he said.

Health concerns,

inconclusive tests

Phosphogypsum contains radium, prolonged exposure to which can lead to a higher risk of lung cancer, and that is why the EPA banned phosphogypsum for use in construction in 1989.

Dr. Paul Papanek, a board member of the Western Occupational and Environmental Medical Assn., said the health effects of contact with phosphogypsum are not immediate. Medical studies about how often the substance causes cancer are inconclusive, he said.

Chinese building-material managers say they have seen an increasing number of drywall makers mixing phosphogypsum in production. They said the corrosion of coils and metals seen in American houses was consistent with drywall made with that ingredient.

For similar reasons, a top manager at Tai'an Single Mechanical & Electrical Technology Co., a supplier of gypsum-processing equipment in Shandong province, also suspects phosphogypsum as a root cause.

The manager, who spoke on the condition of anonymity, estimates that 80% of Chinese drywall makers use phosphogypsum because it is cheap and there are no government restrictions.

Gypsum drywall in China is used largely for businesses, and industry associations say there have been few complaints in China like those from American homeowners.

So far, tests in the U.S. of Chinese-made drywall used in American homes have not turned up evidence of phosphogypsum. In Florida, four samples taken from troubled houses showed no indication of radium, said Lori Streit, a scientist at Unified Engineering Inc., which conducted the analysis.

Streit said the rotten-egg-like odor and corrosion are associated with volatile sulfuric acids, and some industry officials say that could mean the drywall was made with gypsum from mines in eastern China's Tai Mountain area, where ores have unusually high levels of sulfur compounds.

Knauf, a German company that has a joint-venture operation in China and has been a primary target of lawsuits over bad drywall, has acknowledged using gypsum from that area.